There is a wheel hub, the entirety of which is integrally molded by aluminum die casting or the like, disclosed in Japanese Unexamined Patent Publication No. Hei 11(1999)-321206. The wheel hub includes a hub cylindrical portion through which an axle is passed and a pair of flanges on the left and right which are provided at opposite end portions of the hub cylindrical portion along the axial direction. A sprocket for driving a chain and a brake disc are respectively fastened to one flange with bolts, and lightening portions which are open to the outside are provided in side surfaces of the flanges.
Further, there is also a wheel hub disclosed in Japanese Unexamined Patent Publication No. Hei 4(1992)-328001 in which a hub cylindrical portion and left and right flanges are separately formed, in which axle insertion portions are provided at rotation center portions of the flanges, and in which lightening portions are provided in inner portions thereof. Note that, in the present application, the axial direction of a hub and flanges means the direction of the rotation axis of the hub.
Incidentally, of the above-described pair of flanges on the left and right, the sprocket flange to which the sprocket is attached is a portion to which driving force is transmitted. Since driving force is usually larger than braking force, the sprocket flange needs to be given sufficient flange strength, and is thick-walled compared to the brake flange on the opposite side.
Because of this, the total weight of the hub tends to be large. Accordingly, a lightening portion is provided to realize weight reduction. In order to realize further weight reduction, it is desirable that the lightening portion be provided in a more inner portion of the flange.
However, the shape of such a lightening portion is limited to a recessed shape which is open to the outside in the axial direction. Accordingly, there are natural restrictions on the shape of the lightening portion, and it has been difficult to freely form the lightening portion to have an optimum shape for obtaining needed strength and to realize sufficient weight reduction.
Further, if flanges are formed separately from a hub cylindrical portion as in the aforementioned known example, a lightening portion can be provided in a more inner portion of the sprocket flange. However, the shape of the lightening portion remains limited to a recessed shape which is open to the outside in the axial direction. In addition, the whole of the wheel hub cannot be formed integrally. However, in terms of manufacturing efficiency, it is desirable that the whole can be formed by one-time molding in a single step. Furthermore, it is also desirable that molding can be performed without using complex dies including a core or the like. An object of the present invention is to realize such demands.